Report on the WPI Conference: "New Practical Examples for General Chemistry" H. Beall Worcester Polytechnic Institute, Worcester, MA 01609 General chemistry courses are faced with the challenges of beine current and interesting. In addition, i t is important, when serving the majors of ocher disciplines such &biology or eneineerine, that the course content have relevance to the othe;discipl&es, The examples used to demonstrate chemical principles are very important in accomplishing these ends. Practical examples for use in teaching general chemistry was the topic of the Fourth Annual Conference on Chemical Education held March 3, 1990, a t Worcester Polytechnic Institute ( I ) . The speakers were Edward Stiefel of Exxon Research and Engineering Company and Gary Wnek of Rensselaer Polvtechnic Institute. Stiefel stressed that striking a balance between descriptive chemistry and chemical principles is best achieved if the descriptive aspects of chemistry are used to demonstrate the orincioles. He nresented some current biological and envi;onm&al topics that are useful examples forgeneral chemistry courses. The examples were drawn from deep sea hydrothermal vents and the Exxon Valdez oil spill and the biolom and chemistry associated with these. Each of these petroleum chemistry. In each case, examples is related microorganisms play a key role by utilizing a disequilibrium, process with a demonstrathat is, &I avail~bl~spon&neous ble negative free energy change. The raw material out of which oetroleum forms (2) is by photosynthesis in which solar energy drives the conversion of carbon dioxide and water t o biomass and oxvgen. In the predominant cycle, the oxygen is recomhined with the biomass with a spontaneous return to carbon dioxide and water. However, a small fraction of the biomass is buried, and, as it reaches elevated temperature and pressure over large time scales, ultimately yields fossil fuels such as petroleum. Obviously, the photosynthetic oxygen corresponding to this biomass is not used up and accumulates in the atmosphere. The existence of the fossil fuel and the available oxygen represents a disequilibrium that can be taken advantage of as an energy source. This is accomplished when the fuel is comhusted but can also be accomplished hy microorganisms. In either case, carhon dioxide and water are the ultimate products. Stiefel states that for many such disequilihria there are organisms that can take advantage of them. Deep sea hydrothermal vents (3) are found a t subduction zones and spreading centers in the Earth's crust where movement of the plates allows contact of the seawater with hot magma. The vents generally occur a t depths of two or more kilometers. The extreme pressures a t these depths permit liquid water to exist a t very high temperatures with 350 OC being a not uncommon value for water being discharged from a vent. The discharged water is highly concentrated in inorganic salts, which largely precipitate upon contact with the surrounding water, giving characteristic black and white %nokers". 'Massive deoosits of metal sulfides. -~ particularly of manganese and iron, are found associated with current and former vents. In the functioning of the ~
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vent, seawater permeates the ocean floor and contacts the hotrocks. At thisstape, (1) sulfate ions are reduced tosulfide ions, (2) bicarbonate ions apparently converted to carbon dioxide, carbon monoxide, and methane, and (3) metals in the seawater and the rocks are exchanged. In particular, the magnesium ions in the water (0.13% hy weight) are exchanged essentially completely for other ions such as manganese, iron, and calcium. This agrees with the notion that the hard acid Me2f renlaces softer acids in the basaltic subsurface magma.lWhitk smokers are the result of the high concentration of calcium ion in the vent water precipitating upon contact with dissolved sulfate in the seawater. The black smokers and the massive metal deposits result from precipitation of FeS, MnS, and MnOz in the vent water precipitating on cooling. Highly productive biological communities (4) exist in the lightless vicinities of the vents including previously unknown species of tuhe worms, mussels, clams, crabs, and bacteria. The bacteria form the base of the food chain for the other organisms a t the vent. Their source of energy is dissolved H2S,which they oxidize utilizing an existing chemical disequilibrium. Some vents are surrounded by sediment that is rich in petroleum (5) that can he shown by I4C dating techniques to have been formed extremely recently: less than 5,000 years comnared to ordinarv ~etroleum,which is millions of years o l d . ~ h em e ~ h a n i isf~this petioleurn formation involves sedimentation of algal and terrestrial biomass into the heat and pressure of thevent. Thus the vents provide a working laboratory for petroleum formation. The petroleum produced is not observed a t the water surface since microorganisms in the water utilize the available free energy of the oetroleum oxidation before the dronlets can rise to the too. Valdez oil spill The continuing cleanup of the vides another examole of microoreanisms utilizine the disequilibrium and frek energy availible from the oxrdation of netroleum (6).Three methods of cleanuo were oossible, physical, chemical, and biological. ~ h y s i i a lcleanup with hoses and water deluges was performed where possible; chemical treatments including the use of dispersants and heach cleaners were not permitted: biological cleanup was used extensively. .~icroo;~anisms capable of oxidizing petroleum were already present in Prince William Sound as a result of hydrocarbons available from natural seeps, treated ballast water, small spills and run-offs, and terpenes from pine forests. The microorganism population a n d t h e rate of consumption of the petroleum by them was found to be limited by nitrogen and phosphorus nutrients in the water. Special oleophilic fertilizers that would cling to the petroleum residues were soraved on soiled beaches. Removal of the oil from the top layers of beaches treated in this way was dramaticallv evident within two or three weeks. Treatment of 70 milesbf heach was accomplished with a total of 150 tons of the oleonhilic fertilizer. Stiefel concluded with the important generkization that organisms can be expected to he available to take advantage of many of the chemical
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disequilibria that exist on a continual basis on Earth. Wnek presented examples of chemical principles drawn from materials science. He stressed the connection between the prooerties of materials and the chemistry involved and desckbed experimental efforts now in progress a t Rensselaer Polytechnic Institute in the teaching of a materials based chemistry course. This course is a joint effort of the Departments of Materials Engineering and Chemistry a t RPI and is taught by Wnek and Peter Ficalora. Wnek noted that chemists tend to concentrate on the microscopic aspects of matter (atomic and molecular structure) and materials scientists concentrate on macroscopic aspects (bulk properties). He advocated a movement into the area between these extremes. Wnek stated that, particularly in engineering colleges, chemists are under considerable pressure to make chemistry more relevant to enpineering students. In general this means less concentration 'on the iraditional co&s of gases and liquids and more concentration on solids. Failure to respond to these pressures could, in some cases, result in substitution of aeneral chemistry taught by chemists by materials science taGght by engineering faculty. In general, incorporation of more solid state topics and inaterials science into general chemistry is a desirable goal. In the study of solids in general chemistry, Wnek observed. defect-free crvstals receive almost all of the attention. However, it is the defects that result in many of the observable orunerties. Thus the observed strenah of solids is only a fraction of the theoretical calculations assuming no defects. As an example, glass is weakened by the presence of moisture (7). This is the result of water hydrolyzing Si-0 bonds in microcracks in the glass structure. The presence of accessible d orbitals in silicon allows for the formation of pentacoordinate intermediates to facilitate the hydrolysis. Bond strain within the microcracks causes the hydrolysis to take place preferentially there and not on a smooth glass surface. Wnek sueeested that this nrovides a narticularlv real and interesting example of reactivity resulting from bond strain. Potentiallv reactive but larger molecules such as aniline cause much less weakening of glass because of steric hindrances in reaching the strained bonds in the microcracks. Vacancies in metal crystals are important in metal behavior because they allow for solid diffusion. The driving force for the existence of vacancies is the increase of entropy resulting from them. Properties of polymers depend on a number of chemical phenomena. Molecular weight is important as can be shown by trying to pull a fiber from molten wax, which will not work, and from molten polyethylene from a six-pack harness, which will (8).Entropy effects on melting points can be illustrated in the higher meiting point of nylcjn;han polyethylene ( 9 ) .In nvlon, hydrogen bonds in the melt restrict the possible orientations of the molecules, whereas in polyethylene more orientations are possible. The flexibility of poly-
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ethylene is the result of liquid regions between the ordered crystallites. The strength of the material is the result of polymer chains holding the crystallites together. Below -80 OC, the liquid regions of polyethylene undergo a glass transition and the material becomes stiff and brittle. Semiconductors provide a useful example for molecular orbitals with conduction in a Dure solid like silicon beine the result of electron transitions-from the valence band (occuoied molecularorbitals) to themnduction band (unoccunied kolecular orbitals). ~ o b i the n ~silicon with borbn results in the more electronegative boron atom pulling an electron from a neighboring silicon atom and creating a mobile positive charne that can migrate from silicon atom to silicon a p-type se~iconductor.Introduction of an aratom effect and results in an nsenic atom oroduces the oooosite .. type semic&ductor (10). Vacancies in the pure silicon structure allow penetration of the crystal by the doping atoms. Le Chatelier's principle can be invoked to explain the rectifying nature of a p-n junction. In the fabrication of silicon semiconductor devices, the passivating SiOncoating on the silicon must be removed with H F before doping can occur. The strong Si-F bond facilitates this removal. Regrowth of the coating after doping nrotects the device. Wnek concluded by stating the need to merge the viewooints of the chemist and the materials scientist in order to produce interesting relevant general chemistry courses, particularly for the education of engineers. In the discussion that followed, it was asked whether the incorporation of materials science in chemistry were not an erosion of chemistry instigated by engineering faculty. Wnek replied that the RPI approach was simply a matter of changing the examples while still teaching the same principles. For example, steel corrosion can be used to demonstrate oxidation-reduction rather than balancing random redox reactions. Wnek closed by noting that materials science, particularly polymer science, is actually what the majority of chemists on the job are involved with. Acknowledgment The author is indebted to the Educational Development Council of Worcester Polytechnic Institute and Exxon Research and Engineering Company for support.
Literature Clted 1. Beall. H.:Berks, L. H.J. Chsm.Edur. 1980,67,10%LO4. 2. T i s o t , B. P.; Welte. D.H. Petrolmm Formofion and Occurrence, 2nd ed.: Springer: New York, 1884. 3. Corliss, J. D. et al. Science I978,203. 1073. 4. Grade. J. F. Scienn 1985,229. 713: MottleM J. Scienre 1885,229, 717. 5 . Kawka, 0. E.: Sirnoneit, B.R.T. Org. Caochem. 1887,I1.311-328. 6 . Atlas. R.M., Ed. Pefroiaum Microbiology; MacMiIIsn, New Yark, 1984. 7. Michs1ske.T. B.C. Sei. Am. 1887.257 122. . A.:Bunker. . 3. Sperling, L. H. 1ntrodur;ion to Physical ~ ~ i y r nScience; & Wiley: New York, 1886: Chapter I. 9. Billrneyor, F.W. Testbook ofPoiymsrScience. 3rded.; Wiley-lntencienee: Naw York, NY. 198%Chapter 12. 10. Rolssinsky,D. R.J. ChemEduc. 1876.53.617.
